To check the tightness of a fluid-filled vessel, the vessel is charged with an underpressure or an overpressure. The vessel can, for example, be part of a tank system of a motor vehicle having an internal combustion engine. In the event that a leakage is present, fluid flows through the locations of non-tightness to the vessel or out of the vessel whereby the underpressure or overpressure is reduced. This underpressure reduction or overpressure reduction functions as an index for the size of the leak.
For example, in the tank system of a motor vehicle having an internal combustion engine, a mixture, especially of fuel vapor and air, is drawn from the tank-venting system by suction because of an underpressure in an intake manifold of the engine. For this purpose, a tank-venting valve is opened between the intake manifold and the tank system. To prevent fresh air from flowing after into the tank system, a fresh air feed line of the tank system is closed by a check valve. The tank-venting valve is thereupon closed and the underpressure in the tank system is detected by a pressure sensor mounted in the tank system. In the event a leakage is present in the tank system, ambient air can flow into the tank system which leads to a reduction of underpressure.
A method for checking tightness of a tank system of a vehicle is disclosed in U.S. Pat. No. 6,460,518. In this method, the underpressure is built up by means of a pump.
Elastic components of the tank system and especially the tank itself can be deformed because of the underpressure or overpressure whereby the underpressure or overpressure is reduced in the tank system. This operation is characterized also as a creep operation and takes place only once for each component during an underpressure build up or overpressure build up.
With a build up of the underpressure, a part of the fluid disposed in the vessel (especially of a fuel) passes from the liquid phase into the gaseous phase in order to compensate for the quantity drawn off in the gaseous phase. The outgassing likewise leads to a reduction of underpressure. The outgassing reduces with increasing vaporized fluid quantity up to reaching a pressure equilibrium. This outgassing takes place in dependence upon a fluid vapor pressure and a partial pressure of the gaseous fluid.
When charging the vessel filled with the fluid with an underpressure or overpressure, the outgassing or condensation of the fluid disposed in the vessel and/or the deformation of elastic components of the vessel lead to a reduction of the underpressure or overpressure whereby the accuracy of the tightness check is limited.